This article explores the fundamentals of the ERC-20 token standard on the Ethereum blockchain, outlining its concept, history, functions, benefits, and limitations. It addresses the need for token interoperability, security, liquidity, and ease of use for developers and investors. The discussion covers popular ERC-20 tokens like USDT, UNI, and MKR, and compares other ERC standards. Designed for blockchain enthusiasts and developers, it helps understand how ERC-20 simplifies Ethereum-based token and app development. Keywords: ERC-20, Ethereum, token standard, interoperability, security, smart contracts.